This invention relates to disk drive data storage devices, and more particularly to the manufacture of glass substrates used in disk drive data storage devices.
Disk drives for computers store data on a magnetic substance coated on a substrate. The substrates have previously been based upon aluminum, such as AlMg/NiP substrates, which are polished to a smooth finish with an alumina slurry or alumina and silica slurries prior to sputtering with thin film magnetic coatings. The alumina and silica slurries are cleaned from the substrate by the general cleaning mechanisms of mechanical scrubbing, dispersion and etching. Surfactants and pH are generally used for dispersion cleaning, where the surfactant and pH act to separate the slurry particles from each other and from the substrate. Etching is generally accomplished by acids and acid soaps that erode or dissolve the substrate material beneath embedded slurry particles (under-cut) to release them from the substrate. Typical acids in use for NiP plated Al-base substrates include, for example, straight phosphoric acid, nitric acid, hydrofluoric acid-based soaps and phosphoric acid-based soaps. The straight acids generally have a pH less than 1 and the soaps generally have pH""s above 1.
After cleaning, the substrates are sputtered with a series of layers, for example a chrome underlayer, a magnetic layer and a carbon protection layer. If residual alumina particles are left on the substrate, the sputtered layers replicate the irregular surface morphology, creating a bumpy surface on the finished disk. When the head glides over the surface, it crashes into the bumps created by the residual particles that are higher than the glide clearance. This is known as a glide defect, which can ultimately cause file failure. These bumps further cause mag defects, corrosion and decreased disk life. Thus, the residual slurry particles need to be removed from the polished substrate surface so that the substrate is as smooth as possible.
More recently, glass substrates have been used for disk drives in laptop computers. Glass substrates have a higher impact or dent resistance than the aluminum-based substrates, which is important in portable computers where the unit is subject to being bumped, dropped and banged around, causing the head to bang on the disk substrate surface. An additional benefit of glass is that it may be polished to a smoother surface finish than aluminum-based substrates. A smoother substrate allows the head to fly closer to the disk, which produces a higher density recording. Glide height for some computer disk drive files is on the order of 20 nanometers (about 200 xc3x85) and less, which is an extremely small interface distance. Thus, the fact that glass substrates can be polished to smoother finishes makes an industry shift from Al-based to glass substrates desirable, not only for laptop units, but for desktop units as well.
Just as with the aluminum-based substrates, the surfaces of the glass substrate needs to be polished with a slurry to an atomically smooth surface prior to sputtering. It is to be understood that the substrates are relatively thin disks having a top surface and bottom surface, or a Side A and Side B, each of which are polished to a smooth finish. For this polishing process, an aqueous slurry of lanthanide oxides is applied to the substrate. Lanthanide oxides is understood to include oxides of one or more of the rare earth elements of the lanthanide series according to the Periodic Table of Elements, which includes elements 57-71. The lanthanide oxide slurries will typically comprise a major proportion of lanthanum and cerium particles. These slurry particles must subsequently be cleaned off, and this generally is accomplished in a series of steps, including ultrasonic cleaning and mechanical scrubbing (typically referred to as Oliver scrub cleaning) with soap and a pad to remove the loosest slurry.
After these cleaning processes, particles on the order of  less than 0.1 xcexcm (100 nanometers) up to about 1 xcexcm (1,000 nanometers) still remain on the surfaces of the glass substrate. These particles are not easily removed from the substrate, as they are held to the surface by both van der Waals forces, which are very significant at these particle sizes, hydrogen bonding, and molecular bonding of the particles to the surface. Just as with the alumina particles, if these lanthanide oxide particles are left in place on the disk substrate, large glide yield losses and disk corrosion occur in the disk hard file containing the glass substrate, resulting in increased manufacturing costs and customer hard drive failures.
An apparent solution would be the use of acid or base solutions to etch the disk or under-cut the particles similar to that which is done to remove alumina particles from NiP plated aluminum-base substrates. The surface finish of a glass substrate, however, can be damaged by such a method due to low resistance of the glass material to acid etching or overly aggressive acid solutions, such as hydrofluoric acid and caustic etching at high pH""s and temperatures. Damage and compositional change to the polished glass surface will adversely affect the morphology of layers deposited by subsequent sputtering processes and can cause magnetic, glide and corrosion failures. Dissolving off the slurry particles, however, would not be affected by small particle size or molecular bonding in a negative way as using dispersing for cleaning, or necessarily cause surface damage. But dissolving off lanthanide oxides from a glass surface is not easily accomplished, as lanthanide oxides resist dissolution by many acids. Glass substrates currently available, such as those used in laptop computer disk drives, have very high particle values for both Ce and La, which are left from the polishing slurry. For example, some currently available 95 mm aluminosilicate glass substrates contain on the order of 7-58 nanograms (ng) cerium oxide and 15-102 ng lanthanum oxide per substrate. It has been discovered that low Ce and La particle values are critical for low glide heights (xe2x89xa620 nm currently) and near contact recording, so the currently high particle values are unacceptable. It has also been discovered that these particles are a major factor in glide failures where bumps from particles result in removal of the protective carbon layer and subsequently spot corrosion. Thus, lanthanide oxide particle levels must be kept low on glass substrates to achieve the corrosion resistance and smooth surface necessary for use of glass substrates in computer disk drives.
If the market trend toward glass substrates in computer disk drives is to succeed, a cleaning method other than the known acid or base etching techniques is required for removing residual lanthanide oxide particles from the slurry polish that adhere to the surfaces of the glass substrates without altering the polish finish or surface stability to corrosion.
The invention addresses these and other problems associated with the prior art by providing a method for cleaning glass substrates that have been polished with lanthanide oxide slurries.
In an exemplary embodiment, glass substrates which have been polished with lanthanide oxide slurries are cleaned after polishing by immersion in an acid bath of nitric acid, hydrogen peroxide and an organic acid having a carboxylic acid group. The glass substrate may also be further subjected to PVA scrubbing in a basic solution of pH between about 9 and about 12 and immersion in a basic bath of potassium hydroxide of pH between about 11.5 and about 13. In an exemplary embodiment, a glass substrate has polished surfaces with less than about 1.52xc3x9710xe2x88x924 ng/mm2 each of oxide particles of lanthanide series elements. A glass substrate may be produced by the method described above without significantly changing the Al to Si ion surface composition. A disk drive product is also provided comprising a glass substrate having polished surfaces with less than about 1.52xc3x9710xe2x88x924 ng/mm2 each of oxide particles of lanthanide series elements.
These and other advantages and features, which characterize the invention, are set forth in the claims annexed hereto and forming a further part hereof. However, for a better understanding of the invention, and of the advantages and objectives attained through its use, reference should be made to the accompanying Detailed Description, in which there is described exemplary embodiments of the invention.